1. Field of the Invention
The invention relates to a roller mill, particularly for dressing ores, having a rotary grinding bowl surrounded by a grinding area wall and on which roll grinding rollers, an annular area which is bounded by a grinding bowl edge and the grinding area wall, and at least one discharge opening for the comminuted ground material.
2. Related Art
Mills which are used in ore dressing must ensure a careful and gentle comminution. The ground material must only be comminuted to the extent that the following processes for recovering the useful and valuable material of the ore can be performed as efficiently as possible. It is in particular necessary to avoid fines, because they have a negative effect on the following processes.
DE 102 24 009 B4 discloses a shear-free comminution of ores, which uses a LOESCHE-type air-swept rolling mill. The transportation of the ground material within the grinding area and to the classifier, as well as to the downstream dust separator takes place with the aid of an air or gas flow, which flows into the grinding area from an annular duct located below the grinding bowl. As a result of the flow direction from bottom to top and therefore contrary to the force of gravity and the geometrical conditions with respect to the height of the grinding area or the distance from the classifying area, a correspondingly high kinetic energy must be available in order to ensure the pneumatic transport of the ground material particles. The energy costs increase if the size fraction to be transported includes larger ground material particles.
Generally ore dressing requires a coarser size fraction than is required for the cement and power plant industry, where finenesses of e.g. 15% R 0.09 are usual for cement raw material or coal for the downstream burning processes. However, in the case of ores residual values of e.g. 20% R 0.212 are required. This leads to a higher energy demand for the gas flow, so that the known air-swept roller mills can only be used to a limited extent or not at all.
An improved energy balance in the area of particle transport can be obtained if mechanical conveying replaces pneumatic conveying. The ground material fed in the centre of a grinding bowl and which moves outwards as a result of the centrifugal force and is comminuted in the grinding gap between the grinding rollers and grinding surface and then is discharged over the grinding bowl edge, drops into an annular clearance, which is formed by the rotating grinding bowl and a fixed cylindrical housing. Via inclined metal plates in a lower area of the mill the ground material slides to mechanical conveyors, which transport the ground material to separately set up classifying or screening devices, where the fine material is separated from the coarse material. The coarse material is again fed to the mill for further comminution and up to ten cycles are required for obtaining the desired particle size. The fine material is processed in downstream processes.
Such grinding cycles are performed with LOESCHE-type roller mills and are also known in connection with mills of other types, e.g. roller presses (WO 99/54514 A1).
EP 1 247 580 A2 discloses a cyclic grinding device with a high pressure roller mill and a classifier, which are positioned within two fixed side walls and an intermediate, rotary mounted, rotation direction-displaceable material conveying ring for an internal grinding material cycle. In the area below the roller gap of the roller mill is provided a static cascade classifier with V-shaped baffle plates and classifying air is introduced through a feed mechanism housing in one of the side walls into the area below the roller mill. The static cascade classifier acts as a deagglomerator for the roller press scabs or shells. The classifying air containing fines is led out over the side walls, whereas the coarse material particles enter the rotary material conveying ring and are again supplied to the roller mill. A dynamic classifier can also be positioned downstream of the static cascade classifier. This equipment has a relatively significant overall height. Since with both the static and the dynamic classifier the side walls of the operating area form the classifying housing, there are also considerable energy costs.
GB 428 237 discloses a roller mill used for mixing materials and also for comminuting, e.g. ores. The roller mill has, level with the grinding bowl, a discharge opening in the grinding area wall for the comminuted ground material. The horizontal ground material discharge takes place with the aid of fan blades positioned on the underside of the grinding bowl. The grinding process and the degree of comminution are determined by the construction of the housing closely adjacent to the grinding bowl or the grinding area wall, the horizontal discharge opening, the adjustable fan blades and vertical deflectors positioned between the grinding rollers. An alternatively constructed mill has, in addition to the horizontal discharge opening, further openings below the grinding bowl. Into the latter pass the comminuted material which has dropped over the grinding bowl edge and into the annular clearance between the grinding bowl and the lower mill part and it is then mechanically supplied or supplied with the aid of blowers to the further treatment process.